Authors:
Skurtveit, E; Torabi, A, Alikarami, R, Braathen, A.Pages: 3 - 16Abstract: The damage zone of three small faults in the Navajo and Page formations, located on the NE side of San Rafael Swell, Utah, USA, are studied. Scanlines and microstructural analyses are used to document three distinct deformation events: (1) an early phase creating cataclastic deformation bands, during which most of the displacement on the fault took place; (2) a fracturing event with opening and shearing along fractures; and (3) fluid flow and local calcite precipitation along the NW-trending faults. Microstructural characterization of deformation structures shows complex interaction between deformation bands, fractures and the calcite precipitation. Calcite cement is observed as veins in the host rock and along cataclastic bands, with varying amount of cataclastic material floating within the veins. In addition to patchy calcite cement in the host rock, extensive poikilotopic cementation is observed to extend into cataclastic bands with a low degree of cataclasis. However, some cataclastic bands with a high degree of cataclasis show almost no cementation. Development of deformation bands and their link to fracturing affected the flow field, from a fault baffle to a conduit. Calcite cementation reveals the flow paths, before cementation recreated the fault baffle.PubDate: 2015-02-02T06:08:26-08:00DOI: 10.1144/petgeo2014-031Issue No:Vol. 21, No. 1 (2015)

Authors:
Cicchino, A. M. P; Sargent, C, Goulty, N. R, Ramdhan, A. M.Pages: 17 - 34Abstract: Knowledge of compaction behaviour underpins basin modelling and pore-pressure estimation for drilling wells. Mudstones of the Cretaceous Lange and Kvitnos formations at Haltenbanken are diagenetically mature and overpressured with a pressure–depth profile that shows little lateral variation. From density logs, we made the unexpected discovery that porosities vary by a factor of two at depths of around 2700 m below seafloor, with greater porosities in the west, so we investigated possible causes for the variation. Exhumation cannot be the cause because the Cretaceous mudstones are presently at their maximum burial depths across most of the area. Nor are lateral variations in geothermal gradient high enough for diagenesis to be responsible for the lateral porosity differences. X-ray diffraction and grain-size analyses were conducted on cuttings but no significant lithological variations were found. We infer that the lateral differences in compaction trends developed because porewater escape was more inhibited in the west during recent rapid burial by glaciogenic sediments. Associated lateral variations in overpressure may subsequently have decayed. The novel finding in this study is that diagenetically mature mudstones at Haltenbanken display large lateral variations in porosity that cannot be attributed to lateral differences in overpressure, exhumation, temperature or lithology.PubDate: 2015-02-02T06:08:26-08:00DOI: 10.1144/petgeo2014-035Issue No:Vol. 21, No. 1 (2015)

Authors:
van Berk, W; Fu, Y, Schulz, H.-M.Pages: 35 - 42Abstract: Locally increased porosity of carbonate reservoir rocks may result from acidic fluids that migrated as a pre-oil phase through the reservoir. Here, hydrogeochemical modelling, which is based on the principles of chemical equilibrium thermodynamics, is performed to test such a hypothetical concept. Despite the generic nature of the model, the modelling results give basic and quantitative insights into the mechanisms of calcite dissolution in carbonate reservoirs induced by migrating acidic and corrosive aqueous fluids.
The hydrogeochemical batch modelling considers pre-oil-phase aqueous fluids that form by kerogen maturation in siliciclastic source rocks underlying the carbonate reservoir rocks. Although saturated with respect to calcite, migration of such fluids through the carbonate reservoir triggers continuous calcite dissolution along their migration path following a decreasing pressure and temperature regime. One-dimensional reactive transport modelling reveals that thermodynamically controlled chemical re-equilibration among pre-oil-phase fluids, calcite and CO2(g) is the driving force for continuous calcite dissolution along this migration path. This reflects the increasing solubility of calcite in the system ‘pre-oil-phase fluids/calcite/CO2(g)’ with decreasing pressure and temperature. In consequence, such fluids can preserve their calcite-corrosive character, if they are exposed to continuously decreasing pressure and temperature along their migration path through the reservoir.
Supplementary material:
The modelling input files to ensure retraceability of our modelling approach and its results are available at http://www.geolsoc.org.uk/SUP18802.PubDate: 2015-02-02T06:08:26-08:00DOI: 10.1144/petgeo2014-065Issue No:Vol. 21, No. 1 (2015)

Authors:
Gershenzon, N. I; Soltanian, M, Ritzi, R. W, Dominic, D. F.Pages: 43 - 54Abstract: The Victor Unit of the Ivishak Formation in the Prudhoe Bay Oilfield is characterized by high net-to-gross fluvial sandstones and conglomerates. The highest permeability is found within sets of cross-strata of open-framework conglomerate (OFC). These cross-strata are preserved within unit-bar deposits and assemblages of unit-bar deposits within compound (braid)-bar deposits, and may form thief zones limiting enhanced oil recovery. We incorporate recent research that has quantified important attributes of preserved sedimentary architecture into high-resolution models. Waterflooding experiments using these models demonstrate the control that such architecture has on oil production rate, water breakthrough time, and spatial and temporal distribution of residual oil saturation. We found that when the pressure gradient is orientated perpendicular to the palaeoflow direction, the total oil production and the water breakthrough time are larger, and the remaining oil saturation is smaller, than when it is orientated parallel to palaeoflow. The pressure difference between production and injection wells does not affect sweep efficiency, although the spatial distribution of oil remaining in the reservoir critically depends on this value. Oil sweep efficiency decreases slightly with increase in the proportion of OFC cross-strata. Whether or not clusters of connected OFC span the domain does not visibly affect sweep efficiency.PubDate: 2015-02-02T06:08:26-08:00DOI: 10.1144/petgeo2014-017Issue No:Vol. 21, No. 1 (2015)

Authors:
Wu, W; Yue, A, Tong, M, Luo, L, Niu, W.Pages: 74 - 80Abstract: In geochemical elemental logging, the elemental sensitivity factor is a key parameter for converting relative yields of elements obtained from spectral analysis to actual concentrations of the elements in the formation. Here we perform a quantitative analysis of the Monte Carlo N-particle (MCNP) simulated spectra of elements in a model formation and compare the obtained relative yield of each element with its actual dry weight in a model formation to study their relationship. For a series of model formations with simple and complex lithologies, the mixed formation spectrum was obtained through MCNP simulation, and elemental sensitivity factors in these formations were calculated according to elemental sensitivity factor theory and applied in calculating dry weights of actual formation elements. It is thus inferred that the elemental sensitivity factor is a constant quantity and does not vary with the nature of the formation. The elemental sensitivity factors calculated for the simulated formations are close to actually measured sensitivity factors and can be used to analyse actual formation elements.PubDate: 2015-02-02T06:08:26-08:00DOI: 10.1144/petgeo2013-049Issue No:Vol. 21, No. 1 (2015)